The invention concerns a method of compensating offset voltages in a focusing circuit that focuses a beam from a source of light on a recorded medium and/or in a tracking circuit that positions the beam of light on the data-storage tracks on the recorded medium, whereby the beam is reflected from the medium onto a photodetector that consists of several photodiodes and whereby a focusing and/or tracking error signal is derived from the output voltages of the photodiodes by constructing the difference between them.
In equipment for playing back data that can be read out of the data-storage tracks on a recorded medium with an optical pick-up, a beam of light is focused on the recorded medium by a focusing circuit and positioned on the data-storage tracks on the medium by a tracking circuit. The optical pick-up in such equipment--compact-disk players, optico-magnetic equipment for recording and playing back, equipment for recording and playing back DRAW disks, and videodisc players for example--includes a laser diode, several lenses, a prismatic beam divider, a refraction grating, and a photodetector. An optical pick-up of this type is described in Electronic Components and Applications 6, 4 (1984), pages 209 to 215.
Lenses focus the beam of light emitted by the laser diode onto the compact disk, which relfects onto a photodetector. The data recorded on the disk and the actual value for the focusing circuit and for the tracking circuit are obtained from the signal leaving the photodetector. The aforesaid literature calls the actual value of the focusing circuit the focusing error and the actual value of the tracking circuit the radial-tracking error.
The focusing circuit is controlled by a coil. An objective lens travels along an optical axis through the coil's magnetic field. The focusing circuit displaces the lens to ensure that the beam of light emerging from the laser diode is constantly focused on the compact disk. The tracking circuit, which is often called a radial drive mechanism, displaces the optical pick-up radially in relation to the disk, positioning the beam on the spiral data-storage tracks on the disk.
The radial drive mechanism in some equipment consists of a coarse-adjustment mechanism and a fine-adjustment mechanism. The coarse-adjustment mechanism can for example be a spindle that radially displaces the entire optical pick-up--the laser diode, the lenses, the prismatic beam divider, the refraction grating, and the photodetector. The fine-adjustment mechanism radially tilts the beam of light, at a prescribed acute angle for example, advancing the beam slightly along the radius of the disk due to the tilting motion alone.
FIG. 1 illustrates the photodetector PD in the optical pick-up of a compact-disk player wherein three laser beams L1, L2, and L3 are focused on a compact disk. A pick-up of this type is called a three-beam pick-up in the aforesaid reference.
Middle beam L1 is the main beam, and beams L2 and L3 are beams of the +1st and -1st order generated from main beam L1 by a refraction grating.
Photodetector PD consists of four square photodiodes A, B, C, and D arranged in a square. Diagonally opposite the large square comprising photodiodes A, B, C, and D are two other photodiodes E and F, which are also square.
Main beam L1 is focused on photodiodes A, B, C, and D and generates a data signal HF=AS+BS+CS+DS and a focusing-error signal FE=(AS+CS)-(BS+DS). Forward beam L2 is focused on photodiode E and rear beam L3 on photodiode F. Both of these outer beams L2 and L3 generate a tracking-error signal TE=ES-FS. AS, BS, CS, DS, ES, and FS are the photoelectric voltages emitted by photodiodes A, B, C, D, E, and F respectively. Since an astigmatic collimator lens is positioned in the path of the main beam L1 in the optical pick-up, the beam will be circular when precisely focused on the large square that comprises photodiodes A, B, C, and D and will be elliptical when it is out of focus.
FIG. 1a illustrates precise focus and precise tracking, which will be described hereinafter. Since the spot of light produced on the large square by main beam L1 is circular, focusing-error signal FE=(AS+CS)-(BS+DS)=0, and the zero tells the focusing circuit that the focus is precise.
FIG. 1b illustrates imprecise focus deriving from the lens being too far from the compact disk. Focusing-error signal FE=(AS+CS)-(BS+DS)&lt;0, and the negative value tells the focusing circuit that the distance between the lens and the disk is too great. The controls in the focusing circuit accordingly displace the lens toward the disk until focusing-error signal FE becomes zero again.
FIG. 1c illustrates the opposite type of imprecise focus deriving from the lens being too near the compact disk. Focusing-error signal is positive--FE=(AS+CS)-(BS+DS)&gt;0, and the positive value tells the focusing circuit that the lens is too near the disk. The controls accordingly displace the lens away from the disk until focusing-error signal FE becomes zero.
How the tracking circuit operates will now be explained.
The beams L1, L2, and L3 illustrated in FIGS. 1a, 1b, and 1c are precisely on the track, and tracking-error signal TE=ES-FS=0.
FIG. 1d illustrates beams L1, L2, and L3 to the right of the track. Tracking-error signal is negative--TE=ES-FS &lt;0. The controls in the tracking circuit displace the optical pick-up to the left until tracking-error signal is TE becomes zero.
In the opposite situation, shown in FIG. 1e, with the beams to the left of the track, the tracking-error signal becomes positive--TE=ES-FS&gt;0, and the controls in the tracking circuit displace the optical pick-up to the right until tracking-error signal TE becomes zero.
The unobjectionable playback of data--whether audio and video in a videodisc player or audio alone in a compact-disk player--requires, in addition to precise focusing of the beam on the videodisc or compact disk, precise tracking over the disk.
The variable amplifier in the focusing circuit is, however, like any other variable amplifier, affected by an offset voltage at a level that is both dependent on temperature and subject to long-term drift. The offset-voltage drift is caused, along with other parameters in an amplifier by the amplifier aging.
Focusing-error signal FE=(AS+CS)-(BS+DS) is constructed in a differential amplifier. Since the differential amplifier is also affected by an offset voltage and since photodiodes A, B, C, and D emit are not ideal and will emit different voltages or currents at the same light density, the situation is another source of detrimental offset voltages.
To prevent the playback of data from being detrimentally affected by offset voltages, as many offset voltages as possible must be compensated. Compensation, however, can be carried out only approximately by manually adjusted potentiometers because they cannot take changes in the offset voltages due to temperature fluctuations and the aging of specific components into account.